The present invention relates to an improved process for the production of sulfur from hydrogen sulfide. In particular, the present process comprises admixing concentrated oxygen with hydrogen sulfide containing feed gas in a burner and recycling thereto a portion of the reaction product, following the removal of elemental sulfur and water. A scrubbing tower is employed for treatment of a portion of the recycle gas.
The production of sulfur from hydrogen sulfide according to the Claus process is well known in the chemical industry. In this process, a gas mixture comprising hydrogen sulfide, typically a by-product of a petroleum refining plant, is combusted in a burner with oygen, usually in the form of air, to produce sulfur dioxide according to the following reaction: EQU 2H.sub.2 S+3O.sub.2 .fwdarw.2SO.sub.2 +2H.sub.2 O (1)
The effluent from the burner, containing stoichiometric quantities of one volume of SO.sub.2 with two volumes of H.sub.2 S is then passed to a Claus type reactor, where elemental sulfur is produced according to the following reaction: EQU 2H.sub.2 S+SO.sub.2 .revreaction.3S+2H.sub.2 O (2)
Since the reaction is at an elevated temperature, the sulfur product is in its vapor phase and must be liquified in a sulfur condensor. Typically, a series of reactors and condensors are employed to yield a high overall conversion rate.
One disadvantage of a conventional Claus type recovery plant is that the capacity thereof is restricted by the practical pressure limits in the plant, notably by the combustio air blower head and sulfur seal leg depth. The capacity can be increased by replacing air with concentrated oxygen, thereby reducing the flow volume and pressure drop through the system. However, beyond an enrichment level of about 30%, flame temperature limitations of the refractory lining in a conventional burner are exceeded.
Another disadvantage of a conventional Claus type recovery plant is that, due to increasing environmental standards, the yield of sulfur must be maximized, either by using a series of expensive reactors, each successive reactor achieving a decreasing percentage yield, and/or else by employing capital intensive pollution control devices such as present in a conventional tail gas plant, in order to remove unconverted reactants and other pollutants from the off-gas.
To overcome the high temperature problem resulting frm the use of oxygen, it is generally known to recycle to the burner, the low temperature effluent from a later stage of the sulfur recovery plant. This recycle stream serves to dilute and cool the reaction mixture within the burner, thereby controlling the flame temperature.
U.S. Pat. No. 3,331,733 to Venemark discloses a Claus process using a source of oxygen comprising 98% by volume of oxygen and 2% by volume of inert gases. The feed gas entering the Claus plant is cooled and mixed with recycle carbon dioxide containing gas obtained from a Claus combustion furnace system.
U.S. Pat. No. 3,681,024 to Hujsak et al. discloses a method for the production of sulfur from hydrogen sulfide using substantially pure oxygen rather than air, in order to materially decrease the volume of sulfur plant effluent discharged to the atmosphere. The uncondensed fraction separated from the product sulfur is subjected to a condensing or quenching step wherein any free sulfur vapor present in such fraction is removed from the system. The portion of the fraction which remains uncondensed is split into two streams, the larger one of which is recycled to the furnace, and the smaller one which is purged to the atmosphere.
In Oil & Gas Journal, Sept. 30, 1985, pages 39-41, Goar et al. describes the advantages of using pure oxygen in place of air. An 85% increase in capacity was obtained by enriching the air supply to 55% oxygen. Goar et al. discribes recycling a portion of the product mixture, following combustion and condensation and prior to catalytic conversion. The recycle stream acts as a coolant to moderate the reaction furnace temperature, effectively replacing nitrogen as the diluent. Following combustion of the hydrogen sulfide feed gas, a series of three catalytic reactors and condensors are used to recover, respectively, 24.9%, 4.4% and 0.9% of the sulfur contained in the feed gas.
The prior art also discloses in general the use of a scrubbing tower to reduce the escape of reactants and other pollutants into the environment. U.S. Pat. No. 2,413,714 to Keeling discloses a process for producing sulfur in which, following combustion with air and catalytic reaction, the products of reaction are cooled by direct contact with liquid water. The water is at a pressure and temperature above the melting point of the produced sulfur, and the latter therefore condenses to its liquid phase. Thereafter, the gases that are residual to the cooling step are treated in a second direct contact with water, at a temperature lower than the first cooling step, in order to minimize the volume of gases which escape from the reaction system.
The present invention represents an improvement over the prior art in several important respects. The present process does not require catalyst and Claus type reactors to complete conversion of hydrogen sulfide to elemental sulfur. This alone is a considerable savings in equipment and maintenance costs. The present process also permits the use of oxygen, thereby increasing plant capacity and reducing energy consumption. The present process also produces 50% more high pressure steam than processes described in the prior art.
A further advantage of the present invention resides in the use of a relatively high pressure scrubbing tower which is located external to a relatively low pressure recycle loop. This scrubbing tower is designed to complete the reaction of the remaining sulfur gases. The combination of a unpressurized recycle loop, incorporating a water condensor, and a pressurized scrubbing tower, external to the recycle loop, is particularly advantageous because the gas mixture withdrawn from the recycle line, in addition to undergoing further reaction, can be cleaned in the scrubbing tower, and the condensed water from the water condenser can also be cleaned in the same scrubbing tower, offering significant economies, particularly in view of present environmental standards and the relatively high cost of conventional tail gas treatment and pollution control devices.